Regulation of dendritic cells by estrogen receptors during influenza infection Abstract Influenza virus infections result in significant morbidity and mortality in the human population, and the recent H1N1 pandemic underscores the need to understand the physiological and environmental risk factors that regulate the human variation in immune responses to influenza virus. This project will determine how one such physiological factor, estrogen receptor alpha (ER1) signaling, regulates the innate immune response to influenza virus infection in mice. Although humans and rodents often exhibit marked sex differences in the prevalence and severity of infection, little is known about the molecular mechanisms by which these sex differences occur and if they are mediated through effects of sex hormones on the innate immune responses elaborated by monocytes and dendritic cells (DC). The functional importance of newly developed populations of inflammatory and tissue DC in the lung and mediastinal lymph nodes during murine influenza infection has recently been elucidated. We have shown that estradiol/ER1 signaling promotes the GM-CSF-driven inflammatory pathway that leads to development of CD11b+ DC. These DC have features of the tissue and inflammatory DC that are crucial during influenza infection. Our data also show that ER1 signaling modulates the activation and function of mature DC. In this project, we will test the hypothesis that during influenza infection, the de novo development and function of DC are regulated by ER1 signaling in response to normal endogenous levels of estrogens. We will evaluate two distinct mechanisms by which ER1 signaling might influence DC-mediated immune responses during infection in vivo: (1) regulation of de novo DC development from inflammatory monocytes or other DC precursors, and (2) regulation of the functional responses of mature DC. We will address these questions using innovative models that include competitive ER1+/ER1-/- bone marrow chimeric mice and mice in which ER1 deficiency is restricted to DC. These novel models will lead to new insights into how ER1 signaling in response to physiological levels of endogenous estrogens regulates de novo DC development and functional responses during influenza virus infection in both sexes. PUBLIC HEALTH RELEVANCE: Cells of the human and murine immune systems are capable of responding to natural estrogens. We seek to understand how estrogens control the development and function of key regulatory cells of the immune system, termed dendritic cells, during influenza virus infection. This knowledge will determine how natural estrogen levels regulate the immune response during influenza infection and help elucidate why men and women differ in susceptibility to viral infections.